I have Wifi problem before where I had more than 5 devices connected to my unifi router. When new device coming in, it's fail to connect. It's happen to my Samsung Galaxy s and Galaxy Note..but not to notebook and iPhone 3GS.
I had been advised by TMNet call center and XDA forumer to play with the channel and security setting. After a few try, now I don't have the problem anymore. Here is the screen and hope could help your wifi problem.
(Thank firestonez from Lowyatt.net)